1. Technical Field
The invention relates to ultrasound (US) transducer temperature compensation methods, apparatus and programs, and more particularly to US transducers used in automotive occupancy sensing (AOS) systems for sensing the nature or type of occupant and the location of the occupant with respect to the vehicle interior. The system disclosed is useful for other types of signal processing in addition to temperature compensation of AOS ultrasound signals, and may be used in other ranging devices such as cameras, golf or binocular range finders, and measuring devices and instruments.
2. Background Art
Studies have revealed that there is a class of automotive accidents causing injuries associated with airbag deployment and with the nature and position of the vehicle occupant, particularly with respect to airbags deployed adjacent to seats occupied by children or infants in car seats. Automotive occupancy sensor (AOS) systems used in conjunction with airbag deployment systems (ADS) have been developed to regulate the deployment of the airbag, i.e., to determine if deployment is to be aborted, deferred, modified or otherwise controlled in response to the occupancy state of the adjacent vehicle interior. For background on AOS systems see Corrado, et al., U.S. Pat. No. 5,482,314 issued Jan. 9, 1996, and U.S. Pat. No. 5,890,085 which issued Mar. 30, 1999, which patents are hereby incorporated by reference.
AOS utilize various types of sensors which produce signals which provide information relating to occupancy state. Typically electrostatic ultrasound transducers are included in AOS systems as active sensors whereby echoes of ultrasonic signals transmitted by the transducer are detected by the transducer when reflected back from the vehicle interior and occupants.
The electrostatic transducer response is highly temperature sensitive, and vehicle interiors have a large potential temperature range during operation, particularly when driving has just started after some period of non-use in hot or cold weather, and environmental controls such as heaters or air conditioners have not yet moderated extremes of heat or cold of the vehicle interior. The amplitude of the received ultrasound signal (echo signal) using an electrostatic ultrasound sensor may change by more than 200% over a temperature range of xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 F.) to +80xc2x0 C. (+176xc2x0 F.). This change has degrading impacts on a typical AOS occupancy classification algorithm.
One solution which has been proposed is to develop a classification algorithm that is robust against changes in signal amplitude. This approach has been tried with limited success, because in an ultrasound signal for occupant classification, a large amount of the distinguishing information is amplitude related, i.e., a lot of information needed for occupancy classification is present in the amplitude of the ultrasonic echo. Amplitude changes due to temperature rather than due to changes in the state of the occupants or objects being imaged could result in false classifications. It is thus important to keep amplitude variations due to environmental effects small, so that any amplitude variations seen in the echo signal are, reliably, only due to changes in the state of objects or occupants present in the vehicle.
What is needed is a mechanism and method that compensates or normalizes the ultrasound signal with respect to temperature variation such that it appears to the classification algorithm to have been collected at or near a reference temperature at which the classification algorithm is optimized, preferably near room temperature.
This invention includes the following features, functions, objects and advantages in an improved system for AOS ultrasound signal occupancy classification: A system which compensates or normalizes the ultrasound signal with respect to temperature so as to keep amplitude variations due to temperature effects small; and a system which compensates for AOS unit-to-unit variations due to manufacturing tolerances. Other objects and advantages will be evident from the description, drawings and claims.
The US echo signal amplitude compensation system of the invention comprises a hardware/software combination which includes a capacitive divider, a multiplexer, an analog-to-digital converter and microprocessor(s) which include firmware and software to sequence multiplexer switching and to apply compensation algorithms to the US signal. The conventional transducer driver circuitry provides the capacity to transmit US pulses or xe2x80x9cpingsxe2x80x9d and to detect and receive US echo returns produced when the xe2x80x9cpingxe2x80x9d is reflected by objects back to the transducer. The transducer circuitry also produces a transducer output signal which is a function of the voltage across the transducer. The transducer output signal is typically a continuous output both during the pulse transmit phase (transducer operating as a US transmitter) and the echo return receiving phase (transducer operating as a US sensor or detector).
To avoid confusion, the terms xe2x80x9ctransducer signalxe2x80x9d and xe2x80x9ctransducer output signalxe2x80x9d will generally be used herein to refer to the electrical output signal of conventional US electostatic transducer circuitry. The acoustic US transducer output will generally be referred to as a US pulse or ping, and the acoustic echo produced by ping reflection will generally be referred to as an echo return.
Where electrical connections and xe2x80x9clinesxe2x80x9d are described, these refer to conventional means for transmitting and coupling electrical signals. However, except where the context indicates otherwise, signal information in the described embodiments of AOS systems may alternatively be transmitted by other conventional means, such as fiber-optic transmission, wireless data transmission, and the like.
A capacitive divider, which also may be termed a xe2x80x9cvoltage monitorxe2x80x9d, is included in the electrostatic transducer received signal processing circuitry to permit the measurements of changes in the capacitance of the electrostatic transducer circuitry due to temperature changes. The voltage monitor signal is a scaled representation of the transducer output signal, in which the scaling effect may be pre-determined by capacitor selection. The voltage monitor signal amplitude may thus be selected so that the peak amplitude during US transmitted pulse is within the usable range of the AOS electronics. Thus, the capacitive divider subcircuit is an example of a voltage scaling subcircuit producing a scaled output representative of the transducer signal.
The magnitude of the voltage monitor output signal permits the AOS microprocessor and software to utilize real-time transducer sensitivity data to normalize the received signal and optimize the occupancy classification algorithm performance.
The multiplexer alternately selects, as time sequenced inputs, the voltage monitor signal and the transducer signal, and the multiplexer output thus contains both voltage monitor data and echo data, which is digitized by an analog-to digital converter. Software running on the microprocessor(s) of the AOS controls the sequence of multiplexer switching, extracts the digitized voltage monitor and US transducer data, applies the compensation algorithm, and scales the continuing US echo data to remove amplitude variation due to temperature. The amplitude-compensated echo signal is then used by the AOS characterization algorithm software to perform occupancy classification.
A calibration procedure is used to gather data on the temperature dependence of the sensitivity of the particular transducer model selected by the AOS manufacturer. Compensation or scaling parameters are computed from this data and used to adapt the compensation algorithm to the particular transducer characteristics. The AOS unit microprocessor is then programmed to apply the compensation algorithm based on these calibrated scaling parameters to scale US echo signals in operational AOS use, in response to the voltage monitor output data contained in the multiplexed AOS input signal.
In the principal embodiment, the voltage monitor measurement for the transmitted US signal from the transducer is used by the compensation algorithm to scale the received echo US signal that follows the transmitted US signal. The preferred transducers for AOS units are of a type which have substantially similar temperature dependency for both transmitted US sensitivity and received US sensitivity. Preferably, the calibration procedure also includes gathering data on unit-to-unit sensitivity variation due to transducer manufacturing tolerances, circuit board variation and installation effects. The compensation algorithm may optionally include parameters to scale or bias the echo signal to adjust for these non-temperature dependent sources of variation.